The present invention is related to ion implantation. More particularly it is related to a method of reducing downtime for an ion implantation tool. Even more particularly it is related to a method for extending the lifetime of a hot cathode discharge ion source.
Ge+ ion implants have been widely used in the semiconductor industry to pre-amorphize silicon wafers in order to prevent channeling effects. The demands for these pre-amorphizing implants are expected to increase greatly in future semiconductor device manufacturing. The most popular ion feed gas for Ge+ beams is GeF4, because of its stable chemical properties and cost effectiveness. However, very short lifetimes of the hot cathode discharge ion sources, from about 10 to 30 hours, have been observed while operating with GeF4 gas. This compares to a source lifetime of about 150 to 300 hours when BF3, AsH3, or PH3 is implanted. When GeF4, BF3, AsH3, PH3 are implanted for various purposes using the same source in an ion implant tool, the source lifetime is still limited to about 10 to 30 hours of operation with GeF4 itself.
The common source failure mode is that some materials deposit on the cathode surfaces of the hot cathode discharge ion source during extended use of the ion implantation apparatus. This deposition reduces the thermionic emission rate of the source ions from the hot cathode surfaces. The deposition can also cause a short between the cathode and the arc chamber. The short can be a direct short or it can extend along the deposited coating along an insulator. Consequently, the desired arc currents cannot be obtained and the hot cathode discharge sources have to be replaced in order to maintain normal source operation. The short source life increases downtime and greatly reduces the productivity of an ion implanter.
The cause of the short source life in GeF4 ion implantation is believed to be excessive, free fluorine atoms in the ion source due to the chemical dissociation of GeF4 molecules. The arc chamber material is etched away by chemical reaction of the fluorine atoms with the material of the arc chamber. Some of the arc chamber material may eventually deposit on the hot cathode resulting in the degradation of electron emissions from the hot cathode discharge source.
Other implantation gases besides GeF4 are employed in ion implantation and these other gases may also cause shortening of the lifetime of the hot cathode discharge ion source. The term xe2x80x9chot cathode discharge ion sourcexe2x80x9d is used herein to denote any thermionic emission element which when heated to a temperature of at least 1200xc2x0 C. emits electrons. The exact temperature at which electrons are emitted depends on the material of the thermionic emission element.
A typical prior art ion implantation apparatus or tool is illustrated in FIG. 1. Specifically, the prior art ion implantation apparatus comprises an ion source chamber 10 which generates ions to be implanted into a desired substrate. The generated ions are drawn by drawing electrodes 12 and their mass is analyzed by a separating electromagnet 14. After mass analysis, the ions are completely separated by slits 16 and the appropriate ions are accelerated by accelerators 18 to a final energy. A beam of ions is converged on the face of a sample or substrate 20 by a quadrupole lens 21 and scanned by scanning electrodes 22a and 22b. Deflection electrodes 24, 26 and 28 are designed to deflect the ion beam in order to eliminate uncharged particles caused by collision with residual gas.
The ion source chamber 10 is the heart of the ion implantation tool. Five different kinds of ion source chambers are currently known including: a Freeman-type ion source chamber using thermoelectrodes; a Bernas-type ion source chamber; indirectly heated cathode type ion source; microwave type ion source chamber using magnetrons; and RF sources. It should be understood that the terms xe2x80x9cion sourcexe2x80x9d and xe2x80x9chot cathode discharge ion sourcexe2x80x9d are used interchangeably herein.
In order to better understand the present invention, a brief description of a Freeman-type ion source, a Bernas-type ion source and a microwave type ion source is given herein. The other types of ions sources mentioned hereinabove, i.e. indirectly heated cathode and RF, are not illustrated herein, but are also well known to those skilled in the art.
FIG. 2 is a cross-sectional view of a Freeman-type ion source chamber 10. Specifically, in this ion source, plasma is generated by emitting thermoelectrons from a bar-shaped filament 30, an electrical field is generated parallel to filament 30 by an electromagnet 32, a rotating field is caused by filament current, and electrons are moved in the chamber by a reflector 34, thereby improving the efficiency in ionization. The ions generated in the chamber pass through slit 36 and are guided in a direction perpendicular to the filament.
FIG. 3 is a cross-sectional view of a Bernas-type ion source chamber 10 containing molybdenum (Mo) as the main ingredient. The ion source chamber 10 includes a tungsten (W) filament 40 and its opposing electrode 44. The ion source chamber is supplied with the desired gas from gas line 46 and emits thermoelectrons from the filament.
A typical microwave ion source is shown in FIG. 4. Specifically, in this chamber 10, plasma is generated in a discharge box 50 using a microwave caused by magnetron 52. Since this chamber has no filaments, its lifetime is not shortened even by the use of reactive gases. However, metal as well as ions are extracted from the chamber and are attracted to the surfaces of drawing electrodes 54; therefore, a desired voltage cannot be applied or the metal or ions may reach a sample to contaminate it.
Each of the above described ion sources exhibits the problem mentioned herein above. Prior art solutions to the short lifetime problem exhibited by these hot cathode discharge ion sources involve either replacing the hot cathode discharge ion source or coating the interior walls of the ion implantation apparatus with a material that is resistant to chemical attack. The latter solution is described, for example, in U.S. Pat. No. 5,656,820 to Murakoshi, et al.
Despite the success of such prior art processes, there exists a need to develop a new and improved method of extending the lifetime of hot cathode discharge ion sources. Such a method is needed since the prior art solutions are either too time consuming or add additional operating costs to the overall process. The prior art solution also yields an unwanted contaminant, molybdenum, into the substrate when implanting a BF2 species.
An object of the present invention is to provide a simple, yet cost effective method for extending the lifetime of a hot cathode discharge ion source.
Another object of the present invention is to provide a method for removing material deposited on surfaces within the ion implant tool.
A feature of the present invention is that the lifetime of a hot cathode discharge ion source is extended when fluorine-containing gases, such as GeF4, BF3, and SiF4, are employed as the implantation gas.
An advantage of the present invention is that down time and the cost of ownership of ion implantation tools is substantially reduced.
These as well as other objects, features, and advantages are accomplished by a method of ion implanting a substrate, comprising the step of providing an ion implant tool having an ion source chamber. An implant gas is fed into the ion source chamber. The gas comprises a reactive species. The substrate is implanted. Introduction of the reactive species into the ion implant chamber is stopped. A nitrogen containing gas is introduced into the ion source chamber for a period of time extending after stopping the introduction of the reactive species.
Another aspect the invention is a method of running an ion implantation tool, comprising providing an ion implantation apparatus comprising an ion source chamber. A nitrogen-containing gas and an implantation gas are introduced into the ion source chamber at substantially the same time. The tool is then run for implanting. The implantation gas is then turned off and the nitrogen-containing gas flow is continued.
The method of the present invention is particularly applicable for use in ion implantation apparatuses wherein highly fluorinated gases such as GeF4 are employed as the implantation gas. The term xe2x80x9chighly fluorinatedxe2x80x9d is used herein to denote a gaseous compound which contains more than a single molecule of fluorine. It has been observed that an improvement in the lifetime of the hot cathode ion source can be obtained when a nitrogen-containing gas is used in conjunction with GeF4 source gas. Similar improvements are expected to be observed with other implantation gases.